1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming system.
2. Description of Related Art
There are known some conventional image forming apparatuses such as copiers, each of which includes a lateral alignment unit including a pair of alignment members for correction (alignment) by pushing a sheet inward at its lateral edges across the width orthogonal to the moving direction of the sheet, to correct the deviation (inclination) of the sheet from its moving direction.
Since the alignment members of the lateral alignment unit push both the lateral edges inward across the width of the sheet, a sheet having low stiffness (rigidity) (e.g., thin paper) is deformed and cannot be readily aligned.
To correct the deviation from the moving direction of the sheet for its alignment, there are also known some apparatuses each of which includes a register alignment unit including register rollers for receiving the front edge of the conveyed sheet abutting the rollers for a predetermined period of time to warp the sheet.
Unfortunately, the register alignment unit cannot sufficiently warp a conveyed sheet having high stiffness or rigidity (e.g., thick paper), and cannot readily correct the deviation.
These two types of apparatuses cannot sufficiently correct the deviation of sheets from the moving direction, if the sheets are unsuitable for the apparatus. This trouble may adversely affect the subsequent process.
Against these problems, for example, an apparatus disclosed in Japanese Patent Application Laid Open Publication No. 2008-51890 determines the rigidity of a still sheet lying at a waiting position before the start of register alignment and controls the driving of register rollers and sheet feeding rollers for feeding the sheet to the register rollers, to certainly warp various types of sheets having different thicknesses.
Unfortunately, in the apparatus disclosed in Japanese Patent Application Laid Open Publication No. 2008-51890, a sheet having high rigidity, which is forcibly pressed against the register rollers for its warpage, may be squeezed between the register rollers and may not sufficiently warp.
More specifically, the apparatus disclosed in Japanese Patent Application Laid Open Publication No. 2008-51890 cannot appropriately correct the deviation from the moving direction caused by physical parameters, such as the thicknesses and the rigidity, of some sheets, and cannot stably convey the sheets to the subsequent processes.